This invention relates to the preparation of uranium dioxide.
Uranium dioxide (UO.sub.2) is the fuel most commonly used in present day nuclear power reactors. In its final form the UO.sub.2 must meet stringent chemical and density specifications, set by the nuclear industry to allow efficient, economical operation of the power reactors.
The most common method used to obtain the high densities required (&gt;95% theoretical) is by cold pressing the powdered UO.sub.2 into pellets and sintering in hydrogen at temperatures of at least 1600.degree. C. Even under these conditions the UO.sub.2 powder must have a very fine particle size before the sintered pellets will meet the density specification.
There are a number of different methods in use for producing UO.sub.2 of very fine particle size. The method most commonly used is by hydrogen reduction of ammonium diuranate, (ADU), which is produced by ammonia precipitation from a solution of uranyl nitrate or uranyl fluoride. The ADU formed in this manner has a very fine particle size, which carries through to the UO.sub.2. The effluent by-product formed is either ammonium fluoride or ammonium nitrate, which is soluble and is removed in the filtrate. Most other methods also involve formation of effluent by-products which must be disposed of. One method which does not produce any aqueous effluent involves hydrogen reduction of milled UO.sub.3 powder to produce UO.sub.2 of the desired particle size. An alternative to this is wet or dry grinding of the UO.sub.2 after reduction.